Earthquake swarm: NZ just tasted a ‘regional source’ tsunami: What are they?

New Zealand just had a taste of a “regional-source” tsunami – and one that could have easily been much more threatening, under different circumstances.

At 8.28am this morning, near Raoul Island, some 1000km away from New Zealand, an 8.1 quake erupted on fault plane at a relatively shallow depth of 20km.

That quake was most likely triggered by a 7.4 event at the same location that unfolded with the same type of “reverse” faulting – and which in turn may have been influenced by an early morning 7.3 quake off the East Cape.

The jolt was big enough to knock out the tidal gauge on Raoul Island, which hadn’t experienced a shake measuring larger than 8.0 in 43 years.

The 1976 8.2 quake was also preceded by a slightly smaller, but still large 7.8 event.

While today’s quake would have caused serious shaking on Raoul Island, on the water near the epicentre, the resulting tsunami would have been barely noticeable, said Dr Jose Borrero, of Raglan-based marine consultancy eCoast Ltd.

“It would have been in the order of tens of centimetres. But over a space of hundreds of kilometres, if you’re sitting on a boat in the ocean, you might feel some shockwaves coming from the earthquake itself.”

While large and shallow “thrust” quakes have had catastrophic consequences – notably in Japan in 2011 and the Indian Ocean in 2004 – Borrero said this morning’s event “wasn’t that big of an earthquake” when it came to generating ocean tsunamis.

“Of course it would have been big for people on the ground, if it had struck right beneath you, and it would have been really big had this happened directly off Gisborne,” he said.

“But in terms of trans-oceanic tsunamis, and their travel distances, you don’t really see large ones from something like 8.1.”

Danger from afar

New Zealand – which has experienced about 10 tsunamis higher than 5m since 1840 – isvulnerable to three types of these ocean hazards: near-source, regional, and distant-source.

The highest risk comes from near-source events, which can leave people just minutes to evacuate.

Headline projections in an EQC-commissioned report estimated worst-case scenario impacts from a one-in-500 year event could include 33,000 fatalities, 27,000 injuries and $45 billion of property loss.

One recent simulation suggested tsunami waves – up to 12m high in places – could inundate the coastline within an hour if a “megathrust” earthquake struck here.

One of the most deadly scenarios for Wellington is a two-punch 8.9 earthquake and tsunami in the Hikurangi Subduction Zone, assuming a rupture extending across Cook Strait.

One GNS Science report found this catastrophic event could cause 40 deaths because ofearthquake shaking – but a further 3200 in a tsunami that followed.

Although scientists are uncertain whether a rupture could extend from the subduction zone to the Cook Strait, such an eventuality could send tsunami waves 5m to 10m high barrelling toward the capital.

At the other end of the scale, a distant source tsunami, like one generated from Chile, could take 14 hours or more to arrive.

To cause any great impact to New Zealand, events that far away have to be enormous.

For instance, one of the largest quakes ever recorded – the 9.5 Chile event in 1960, which had a rupture length of several hundred kilometres – triggered a tsunami that measured higher than 10m when it hit Hawaii, but 4m when it finally arrived in New Zealand.

Today’s event was an example of a regional source tsunami.

Even though one of these typically takes between one and three hours to arrive in New Zealand, they still pose a major challenge to monitoring and warning agencies.

“To locate an event, evaluate its tsunami potential and issue a warning in so short a time is problematic, requiring pre-planning and scenario development,” a 2013 GNS Science report into tsunami risk said.

“Self-evacuation of residents will be required at short notice. Regional source tsunami may represent a significant hazard and risk, and these may be catastrophic on rare occasions.”

Sources of these tsunami include earthquakes and volcanoes erupting or collapsing – especially in tectonically active regions to the north of New Zealand, or south from about 50-60°S.

Any regional threats from the east and west of New Zealand are highly unlikely, and our most threatened coasts are those along the northern half of the North Island and the southern half of the South Island.

In New Zealand’s historical record, the largest earthquakes along the arc between Vanuatu, Kermadec Islands and Tonga have been less than 8.5.

And only two of these have been known to cause tsunami with “run-ups” in New Zealand approaching 1m.

To the south of New Zealand, only a few large earthquakes have occurred since the 1960s, when the installation of a worldwide seismic network allowed large earthquakes to be identified and located.

Direction matters

“Anything that happens along the Kermadec Trench affects us less and less the further north it is – and this morning’s earthquake was about 1000km north of us,” Borrero said.

“In that respect, we’re talking about a zone where not a lot of energy gets to New Zealand, relative to what was pushed out from the earthquake.

“So, energy from the tsunami goes out perpendicularly to the fault line. The fault line runs north to south, so the energy goes east to west – and we’re to the south.”

When a 7.4 quake struck in June last year, New Zealand avoided a tsunami because the event didn’t have the right sense of motion to create one.

Yet it was just on the north end of what he called New Zealand’s “hot zone” for tsunami effects, from the Tonga-Kermadec Trench.

From that point south, tsunami energy would be aimed directly at sections of the New Zealand coast – but north of it, most of the energy was sent into the western South Pacific, passing the country.

“That was why the Samoa 2009 tsunami was not so big down here, the tsunami energy is mostly radiated perpendicular to the trench,” Borrero said at the time.

“But once the source region slips south of the latitude of North Cape, the east coasts of Northland, Auckland, Waikato and Bay of Plenty are in the direct line of fire.”

Is it possible for quakes in New Zealand to trigger jolts in that region, that in turn create regional-source tsunamis?

Borrero said the fact today’s East Cape and Raoul Island events came within hours of each other would particularly be cause for investigation.

“Seismologists are gonna be scratching their heads for a while on this one.”

It was too early to say whether the second quake might have come as a result of the fault being “loaded” with stress by the first, but Borrero said it could be a factor.

GNS Science seismologist John Ristau thought it would be difficult to make any link.

“The [different earthquakes] are about 1000km apart, so it’s probably a bit of a stretch to say that the East Cape earthquake caused any kind of stress changes or anything that would have resulted in these ones near Raoul Island,” he said.

“The only thing you might say is that if that 7.4 near Raoul Island was about to rupture anyway, it could be that the seismic waves on the East Cape earthquake, as they were travelling through, moved things around just enough, and that proved the straw that broke the camel’s back to make the 7.4 happen.

“But that would mean the 7.4 was inevitable, or that it was going to happen sometime soon, anyway.”

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